Problem: Michael ate 3 slices of pizza. Brandon ate 5 slices. If Michael ate $\dfrac{3}{13}$ of the pizza, what fraction of the pizza is remaining?
If 3 slices represent $\dfrac{3}{13}$ of the pizza, there must have been a total of 13 slices. $\text{fraction of pizza remaining} = \dfrac{\text{number of slices remaining}}{\text{number of slices total}}$ Together they ate 8 slices, which leaves 5 out of 13 slices remaining. There is $\dfrac{5}{13}$ of the pizza remaining.